A few simple configuration tweaks could save you sleepless nights over whether or not someone might crack your SSH server.

Chances are, if you are the owner or administrator of a Linux machine,
you access it remotely from time to time, if not constantly. Be
it a workstation at home, a co-located server or a hobby machine, if
you are accessing Linux remotely, there is a good chance that you
are using the OpenSSH server on the remote machine with some type of
SSH client locally. (If you are not, you probably should be.) Although
it is true that the OpenSSH server and clients do a tremendous job at
encrypting the traffic that passes between systems, it also is true that
any dæmon listening for connections is a door handle
waiting to be turned by the black hats, the evildoers or the crackers.
The less-desirable folks in these
situations are, as the title states, demons seeking dæmons, a person or
group of people that usually are up to no good seeking listening dæmons
that, depending on the configuration of the machine, may or may not be
secure. Hackers can be defined as people who have relatively benign motives
for breaking into a system. Crackers often are thought of as hackers
with malicious intent. Neither the former nor latter is welcome on my
machines. In this article, I expand
on the basic OpenSSH configuration and cover ways to improve
the security of the SSH dæmon to offer more protection to
your machines. If you have a machine
that suffers a compromise, your machine may be helping to spread your
unpleasantness to others as well.
The Internet is one of the largest insecure data routes on the face of
our planet.

OpenSSH provides the means for secure communication over insecure
channels. The file sshd_config is the ruling party in the game of
secure shell configuration. The sshd_config file consists of multiple
options that can be changed to help improve the security of the listening
dæmon. Although it may seem that because it is a remote access tool
it should be secure right out of the box, this is
hardly the case. For the most part, a default installation of the OpenSSH
server will provide you with a relatively secure default configuration
but one that can be improved substantially by the machine's administrator.

When planning for remote access initially, it is suggested that you
consider
three major things for the access of your machines:

Who will be allowed access to the machines?

How will this access be provided?

From where will this access be allowed?

We are going to assume that
we will be using the OpenSSH server to provide remote access. This
leaves us with the questions of who will be allowed remote access and from where
they will be allowed. For some, this may be simple; perhaps the machine
supports only a single user and remote access is carried out from a
single domain. For others, this can be quite a challenge when multiple
users that travel frequently are the machine's users.

Who Is on the Invite List for This Devilish Party?

The first and foremost person on a Linux machine is the root user. This
is universally known, and it also should be universally known that if you need
to have remote root access to a machine, there are many better ways to
access as root than simply using SSH and logging in as root. If you think
about the fundamentals of a brute-force attack attempt, it is obvious
that the most relevant account that will be attacked is the root
account. One need not guess whether the
account is present; it is there. The sshd_config file lets us
specify that root is not allowed to log in remotely at all through the
PermitRootLogin directive.

I fully believe that the saying “an ounce of prevention equals a
pound of cure”
is highly accurate when dealing with remote-use accounts. Two
options allowed in the sshd_config file, UsersAllow and UsersDeny, are
more than an ounce of prevention, and although it may be one extra step
when adding an account, modifying the UsersAllow for each account added
provides that pound of cure that you may (thankfully) never need to
seek. To expand on the UsersAllow directive, you cannot specify
only particular users, but you also can specify particular users at specific hosts. So, if
in advance you know exactly who will need to log in and from where, the
minimal time overhead associated with adding these directives to the
sshd_config file provides the peace of mind for you to know you are
allowing remote access for only specific accounts from specific machines. Valid
users can be retrieved from the /etc/passwd file in standalone machines
or from the corresponding files in NIS or LDAP environments. Listing
1 is an example of parsing the previous month's security files to verify
which accounts have logged in successfully using SSH.

"DenyHosts is a python program that automatically blocks ssh attacks by adding entries to /etc/hosts.deny. DenyHosts will also inform Linux administrators about offending hosts, attacked users and suspicious logins." Very nifty when your machine is getting hammered with brute force attacks. You can find it here.

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